An outline of ozone treatment of clean water will be described below. Raw material gas (an oxygen gas to which a trace amount of nitrogen gas is added, or air) is supplied to an ozone generator, in a discharge tube inside of the ozone generator, the raw material gas is discharged by a high-frequency high voltage which is supplied from a power supply, and by this discharge, the oxygen gas in the raw material gas is changed to be an ozonized gas. In the ozone generator, heat which is generated by discharge is cooled by cooling water. The ozonized gas which is generated in the ozone generator is fed to an air diffuser from a bottom part of an ozone contact tank; the ozonized gas is fed to water to be treated as small air bubble and is dissolved in water. An organic substance in water to be treated such as an odor component, bacteria, etc. is oxidatively decomposed by ozone. As a result, the water to be treated is discharged from the ozone contact tank as odorless and sterilized water.
A ozone generation amount which is necessary for treatment of an ozone contact tank is determined by quantity of flow of water to be treated, water quality, water temperature, etc. Regarding a method for obtaining necessary ozone generation amount, there is a case in which the necessary amount of ozone is determined by performing experiments in advance, and another case in which dissolved ozone concentration in the ozone contact tank is measured and then feedback is applied. In order to generate necessary amount of ozone which is obtained by the above-mentioned methods, in conventional devices in which an oxygen gas (a trace amount of nitrogen is included) is used as raw material gas, the devices are operated to adjust the flow rate of an ozonized gas with constant ozone concentration.(ozone generation amount)=(ozone concentration)×(the flow rate of an ozonized gas)
A running cost of a device is total of an electricity charge which is consumed by an ozone generator and a gas charge of liquid oxygen. In general, as an ozone concentration increases, efficiency of an ozone generator decreases. Consequently, electricity consumption increases, therefore, an electricity charge increases. On the other hand, as an ozone concentration increases, necessary flow rate of an ozonized gas decreases. Consequently, a gas charge decreases. As a result, it is known such that a running cost is a minimum at a certain ozone concentration. (For example, Patent Document 1, Non-Patent Document 1) An ozone concentration at which a running cost is a minimum is affected by the configuration of a discharge tube of an ozone generator, cooling water temperature, etc.
Further, in Europe and the United States, injectors are commonly used as a device for dissolving an ozonized gas in water and injector pumps are used for feeding water to the injectors. When an ozone concentration increases, a flow rate of an ozonized gas decreases, and an amount of water which flows in an injector decreases. Consequently, an electricity charge of the injector decreases. For example, in some cases, by operating an ozone generator at a higher ozone concentration, a running cost is lower. An ozone concentration which has been commonly used conventionally is 10 wt %, however, for example, in some cases, by operating the ozone generator at concentration 16 wt %, a running cost is lower.
On the other hand, there is a method for improving ozone generation efficiency at a high ozone concentration, for example, a method for shortening a discharge tube gap length (Patent Document 2)